The invention relates generally to electromagnetic clutches for use in motor vehicle drive line components such as transfer cases, transmissions and the like and more particularly to an electromagnetic clutch having a ball ramp actuator and a torsion spring operably disposed between components of the ball ramp actuator.
Electromagnetic clutches utilizing ball ramp actuators are especially suited for use in motor vehicle driveline components. A typical ball ramp actuator electromagnetic clutch utilizes a pair of opposed circular plates defining opposed pairs of symmetrically ramped recesses. Adjacent the circular plates is a friction clutch pack having an input driven by the primary transfer case output shaft and a secondary output to the secondary driveline which is driven by the output of the friction clutch pack assembly. Retardation of motion of one of the circular plates causes axial separation thereof and such axial separation compresses the friction clutch pack and transfers torque from the input to the output.
One of the foremost advantages of a ball ramp actuated electromagnetic clutch assembly is its relatively low power consumption. Since the electromagnetic coil must only retard one of the circular plates to commence engagement of the clutch, the clutch coil and its current draw may be significantly smaller than a clutch wherein the electrical energy is utilized as the sole and direct energy source which engages the clutch.
While such clutch engagement can generally be accurately controlled by the controlled application of electrical energy to the electromagnetic coil, occasionally the deactivation of the clutch may be subject to mechanical and magnetic hysteresis. That is, clutch disengagement may not accurately track or correspond to decreasing clutch current. This may be the result of a failure by the clutch plates to modulatably disengage or may be the result of residual magnetism in the clutch components or other operating anomalies, which cause non-linear or non-proportional disengagement.
The present invention is directed to improvements in the art of ball ramp clutch actuators for electromagnetic clutches which improve the proportional response of such clutches, particularly during deactivation.